well oss is deprecated by alsa. and esd is just a sound daemon used by gnome (and you missed artsd used by kde). a sound daemon is kinda like a traffic cop for the sound system.

the musepack development team wrote and maintains "libmpcdec" which is a library decoder for the musepack format. a sound program (player) or even the sound system itself (gstreamer) would have support for musepack as they utilize the library decoder (libmpcdec) currently on the system.